If you don't have an e-ticket there are two choices: You can either print your ticket off at a booth near the port, or you can use a travel agency. There won't be a fee for using a booth but in some circumstances there may be a small handling fee of one or two euros at a travel agency. The agency can be around the port or eleswhere.
Booths will open before the departure of your ferry and will have the logo of the ferry company you are travelling with. But if you don't see your company, just go and ask, many companies co-operate and merge over time. Also note that in the low season or early in the morning they may open close to the time off your departure, but don't worry, they will open.